Hall thrusters are plasma propulsion devices that have found application on-board spacecraft for stationkeeping, orbit transfers, orbit raising, and interplanetary missions. A unique combination of thrust efficiency, thrust density, and specific impulse makes Hall thrusters qualified to fill such a varied array of missions. Hall thrusters typically operate between 50-60% efficiency, thrust densities of 1 mN/cm2, and specific impulses of 1000-3000 s. Hall thrusters have been flying in space since the 1970s and American designed Hall thrusters began flying in 2006.
Hall thrusters produce thrust by ionizing a propellant, typically xenon, and accelerating the resulting ions by way of the application of crossed electric and magnetic fields. The discharge chamber used to produce the plasma in Hall thrusters has traditionally been employed as a constant cross-sectional area along its axial extent. Variable area discharge chambers have also been sporadically reported in literature but various deficiencies have limited the utility of such Hall thrusters.
An investigation of the dependence of propellant utilization on discharge chamber width was discussed in Raitses, et al., “Propellant Utilization in Hall Thrusters,” Journal of Propulsion and Power, Vol. 14, No. 2, March-April 1998. In this study, the channel width of a low-power Hall thruster was decreased by 40-55% by using a ceramic spacer that was inserted on the outer wall of the channel. The ceramic spacer or insert extended from the midpoint of the axial span between the anode and the peak magnetic field. It was found that the ceramic spacer improved mass utilization and specific impulse while essentially leaving efficiency unchanged except at high current density where the ceramic spacer resulted in decreased efficiency. This work demonstrated how an asymmetric spacer or insert can form a variable channel cross-section and improve propellant utilization. However, the use of such an asymmetrical spacer or insert that did not conform to the local magnetic field and introduced radial asymmetries into the neutral distribution likely limited the realization of the full benefits of the variable chamber width.
To achieve high thrust-to-power operation with a Hall thruster requires operation at low discharge voltages, typically in the range of 100 V to 150 V. This is much less than the 300 V to 500 V range where Hall thrusters typically operate. At such low voltages, the ionization efficiency is significantly decreased because the electron temperature approximately scales with discharge voltage as follows: Te˜0.1 Vd. As a result, at 100 V the electron temperature is approximately only 10 eV which is on the order of the first ionization potential of xenon, 12.1 eV. Therefore, such a low electron temperature leads to poor ionization efficiency, ultimately limiting the thrust-to-power ratio that can be achieved with the Hall thruster.
Accordingly, there exists a need for a Hall thruster that can provide a relatively high ionization efficiency at low discharge voltages thereby achieving a high thrust-to-power ratio at high efficiency. There also exists a need for a Hall thruster that can provide a relatively high ionization efficiency at high discharge voltages.